Without limiting the scope of the invention, its background is described in connection with coupled quantum well logic devices, as an example.
Integrated circuits have become the technology of choice for performing logic functions. The downscaling of minimum device geometries has provided for increases in the functional density and performance of integrated circuits. The development of nanoelectronic devices has allowed for the continuing increase in functional density of integrated electronic systems beyond the currently perceived limits for conventional electron devices. The term "nanoelectronics" refers to an integrated circuit technology that permits downscaling of minimum circuit geometries to the order of 0.01 microns, or 10 nanometers (nm).
In electronic devices having nanometer dimensions, the behavior of electrons can best be understood by considering their wave-like properties. Two important electron quantum phenomena that can be observed are "tunneling", whereby electrons pass through potential energy barriers, and "resonance", whereby steady state tunneling current is substantially reinforced because of the dimensions of quantized regions through which electrons tunnel. Tunneling and resonance are observed when allowed energy states between adjacent quantum well regions are aligned.
When electrons are confined by potential barriers to regions approaching the size of the electron in all three spatial dimensions, the spectrum of allowed energy states for the electron is discretized and the region is called a quantum dot. Similarly, when the electron is confined in two spatial dimensions, the allowed energy spectrum is also modified and the region is called a quantum wire. When electrons are confined by potential barriers in only one spatial dimension, the region is referred to as a quantum well. The term quantized region, as used herein, is a general term that refers to any of those structures.
Physically, quantum dots can be as simple as nanometer-sized cubes or rods of indium gallium arsenide (InGaAs) embedded in a matrix of wider band-gap material such as indium phosphide (InP). The effect of the InP is to create a potential energy barrier for electrons in the InGaAs. These nanometer-sized structures produce sharply defined and well separated allowed electronic energies. An electrical bias can align allowed energy states in potential wells (e.g. quantum dots) which are separated by tunneling barriers, thereby allowing electrons to tunnel through the intermediate barrier material by the process of quantum mechanical resonant tunneling. In this way, the relative electrical isolation or connectivity between quantum dots or wires can be controlled by the electric fields placed on the tunneling barriers. These electric fields serve to either align allowed energy states of adjacent quantum wells (thereby allowing conduction of electrons), or to misalign the allowed energy states so that tunneling does not occur. This control of current through the potential wells by the application of electric potentials can be exploited to build useful electronic devices.
A promising device which makes use of quantum tunneling is the lateral resonant tunneling transistor. In such a device, quantized wells are disposed between non-horizontal physical tunneling barriers such that allowed energy states can be separately modified to enhance or suppress electron tunneling. A method of forming such a device is taught in U.S. patent application Ser. No. 07/787,850, filed Nov. 5, 1991. The method taught involves the epitaxial deposition of continuous films in which trenches are etched to form tunneling barriers.